/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.expression.spel;

import java.util.ArrayDeque;
import java.util.Collections;
import java.util.Deque;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;

import org.springframework.core.convert.TypeDescriptor;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.EvaluationException;
import org.springframework.expression.Operation;
import org.springframework.expression.OperatorOverloader;
import org.springframework.expression.PropertyAccessor;
import org.springframework.expression.TypeComparator;
import org.springframework.expression.TypeConverter;
import org.springframework.expression.TypedValue;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;

/**
 * An ExpressionState is for maintaining per-expression-evaluation state, any changes to
 * it are not seen by other expressions but it gives a place to hold local variables and
 * for component expressions in a compound expression to communicate state. This is in
 * contrast to the EvaluationContext, which is shared amongst expression evaluations, and
 * any changes to it will be seen by other expressions or any code that chooses to ask
 * questions of the context.
 *
 * <p>It also acts as a place for to define common utility routines that the various AST
 * nodes might need.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 3.0
 */
public class ExpressionState {

    private final EvaluationContext relatedContext;

    private final TypedValue rootObject;

    private final SpelParserConfiguration configuration;

    @Nullable
    private Deque<TypedValue> contextObjects;

    @Nullable
    private Deque<VariableScope> variableScopes;

    // When entering a new scope there is a new base object which should be used
    // for '#this' references (or to act as a target for unqualified references).
    // This ArrayDeque captures those objects at each nested scope level.
    // For example:
    // #list1.?[#list2.contains(#this)]
    // On entering the selection we enter a new scope, and #this is now the
    // element from list1
    @Nullable
    private ArrayDeque<TypedValue> scopeRootObjects;


    public ExpressionState(EvaluationContext context) {
        this(context, context.getRootObject(), new SpelParserConfiguration(false, false));
    }

    public ExpressionState(EvaluationContext context, SpelParserConfiguration configuration) {
        this(context, context.getRootObject(), configuration);
    }

    public ExpressionState(EvaluationContext context, TypedValue rootObject) {
        this(context, rootObject, new SpelParserConfiguration(false, false));
    }

    public ExpressionState(EvaluationContext context, TypedValue rootObject, SpelParserConfiguration configuration) {
        Assert.notNull(context, "EvaluationContext must not be null");
        Assert.notNull(configuration, "SpelParserConfiguration must not be null");
        this.relatedContext = context;
        this.rootObject = rootObject;
        this.configuration = configuration;
    }


    /**
     * The active context object is what unqualified references to properties/etc are resolved against.
     */
    public TypedValue getActiveContextObject() {
        if (CollectionUtils.isEmpty(this.contextObjects)) {
            return this.rootObject;
        }
        return this.contextObjects.element();
    }

    public void pushActiveContextObject(TypedValue obj) {
        if (this.contextObjects == null) {
            this.contextObjects = new ArrayDeque<>();
        }
        this.contextObjects.push(obj);
    }

    public void popActiveContextObject() {
        if (this.contextObjects == null) {
            this.contextObjects = new ArrayDeque<>();
        }
        try {
            this.contextObjects.pop();
        }
        catch (NoSuchElementException ex) {
            throw new IllegalStateException("Cannot pop active context object: stack is empty");
        }
    }

    public TypedValue getRootContextObject() {
        return this.rootObject;
    }

    public TypedValue getScopeRootContextObject() {
        if (CollectionUtils.isEmpty(this.scopeRootObjects)) {
            return this.rootObject;
        }
        return this.scopeRootObjects.element();
    }

    public void setVariable(String name, @Nullable Object value) {
        this.relatedContext.setVariable(name, value);
    }

    public TypedValue lookupVariable(String name) {
        Object value = this.relatedContext.lookupVariable(name);
        return (value != null ? new TypedValue(value) : TypedValue.NULL);
    }

    public TypeComparator getTypeComparator() {
        return this.relatedContext.getTypeComparator();
    }

    public Class<?> findType(String type) throws EvaluationException {
        return this.relatedContext.getTypeLocator().findType(type);
    }

    public Object convertValue(Object value, TypeDescriptor targetTypeDescriptor) throws EvaluationException {
        Object result = this.relatedContext.getTypeConverter().convertValue(
                value, TypeDescriptor.forObject(value), targetTypeDescriptor);
        if (result == null) {
            throw new IllegalStateException("Null conversion result for value [" + value + "]");
        }
        return result;
    }

    public TypeConverter getTypeConverter() {
        return this.relatedContext.getTypeConverter();
    }

    @Nullable
    public Object convertValue(TypedValue value, TypeDescriptor targetTypeDescriptor) throws EvaluationException {
        Object val = value.getValue();
        return this.relatedContext.getTypeConverter().convertValue(
                val, TypeDescriptor.forObject(val), targetTypeDescriptor);
    }

    /*
     * A new scope is entered when a function is invoked.
     */
    public void enterScope(Map<String, Object> argMap) {
        initVariableScopes().push(new VariableScope(argMap));
        initScopeRootObjects().push(getActiveContextObject());
    }

    public void enterScope() {
        initVariableScopes().push(new VariableScope(Collections.emptyMap()));
        initScopeRootObjects().push(getActiveContextObject());
    }

    public void enterScope(String name, Object value) {
        initVariableScopes().push(new VariableScope(name, value));
        initScopeRootObjects().push(getActiveContextObject());
    }

    public void exitScope() {
        initVariableScopes().pop();
        initScopeRootObjects().pop();
    }

    public void setLocalVariable(String name, Object value) {
        initVariableScopes().element().setVariable(name, value);
    }

    @Nullable
    public Object lookupLocalVariable(String name) {
        for (VariableScope scope : initVariableScopes()) {
            if (scope.definesVariable(name)) {
                return scope.lookupVariable(name);
            }
        }
        return null;
    }

    private Deque<VariableScope> initVariableScopes() {
        if (this.variableScopes == null) {
            this.variableScopes = new ArrayDeque<>();
            // top-level empty variable scope
            this.variableScopes.add(new VariableScope());
        }
        return this.variableScopes;
    }

    private Deque<TypedValue> initScopeRootObjects() {
        if (this.scopeRootObjects == null) {
            this.scopeRootObjects = new ArrayDeque<>();
        }
        return this.scopeRootObjects;
    }

    public TypedValue operate(Operation op, @Nullable Object left, @Nullable Object right) throws EvaluationException {
        OperatorOverloader overloader = this.relatedContext.getOperatorOverloader();
        if (overloader.overridesOperation(op, left, right)) {
            Object returnValue = overloader.operate(op, left, right);
            return new TypedValue(returnValue);
        }
        else {
            String leftType = (left == null ? "null" : left.getClass().getName());
            String rightType = (right == null ? "null" : right.getClass().getName());
            throw new SpelEvaluationException(SpelMessage.OPERATOR_NOT_SUPPORTED_BETWEEN_TYPES, op, leftType, rightType);
        }
    }

    public List<PropertyAccessor> getPropertyAccessors() {
        return this.relatedContext.getPropertyAccessors();
    }

    public EvaluationContext getEvaluationContext() {
        return this.relatedContext;
    }

    public SpelParserConfiguration getConfiguration() {
        return this.configuration;
    }


    /**
     * A new scope is entered when a function is called and it is used to hold the
     * parameters to the function call. If the names of the parameters clash with
     * those in a higher level scope, those in the higher level scope will not be
     * accessible whilst the function is executing. When the function returns,
     * the scope is exited.
     */
    private static class VariableScope {

        private final Map<String, Object> vars = new HashMap<>();

        public VariableScope() {
        }

        public VariableScope(@Nullable Map<String, Object> arguments) {
            if (arguments != null) {
                this.vars.putAll(arguments);
            }
        }

        public VariableScope(String name, Object value) {
            this.vars.put(name, value);
        }

        public Object lookupVariable(String name) {
            return this.vars.get(name);
        }

        public void setVariable(String name, Object value) {
            this.vars.put(name, value);
        }

        public boolean definesVariable(String name) {
            return this.vars.containsKey(name);
        }
    }

}
